The invention relates to the discovery of a process integration that interposes a hydrocarbon liquids production process between a fossil fuel-based synthesis gas production process and an ammonia production process in a manner that substantially improves the economics of the overall integrated process. The invention particularly relates to the discovery of a means to enhance the economic feasibility of relatively small scale, syngas based ammonia fertilizer production plants by the integration of compatible liquid hydrocarbon synthesis processes into the ammonia production plant complex utilizing a portion of the fossil fuel-based syngas production as a feedstream to the hydrocarbons production process
The production of ammonia fertilizers by the reduction of nitrogen gas with hydrogen as derived from synthesis gas (syngas) is an established process, well known in the art. The process is of great importance to developed as well as developing countries with the population of ammonia fertilizer plants, both small and large, high in all cases. Typically, the required synthesis gas is produced by partial oxidation or steam reforming of fossil fuels such as coal or natural gas by processes well known and established in the art. When it is readily available, natural gas, of course, is by far the preferred syngas feedstock due to its higher hydrogen to carbon ratio, ease of transportation and cleanliness.
For those countries with an abundance of natural gas reserves, particularly those in the Middle East, the production and export of ammonia fertilizer is a major industry. Very large complexes of ammonia production based on natural gas derived syngas have been built there that enjoy all the economic process advantages attendant upon great plant size and large scale production. As an export commodity, the ammonia fertilizers they produce are exceedingly price competitive, beyond the means of the small, native ammonia plants of importing countries to match, even when their raw material is natural gas. However, the available fossil fuel for these countries is often coal. Ammonia produced from coal-derived syngas is appreciably more costly than an equivalent natural gas/syngas ammonia process. Consequently, the native ammonia fertilizer industry of those importing countries is threatened with native plant obsolescence and long-term dependence upon imported ammonia fertilizer. Hundreds of relatively small, coal-based native ammonia fertilizer plants lie threatened with closure in the face of import competition unless those small plant installations, can be altered to improve the overall plant economics of operation for ammonia production from coal over the economics of imported ammonia fertilizer produced from natural gas. Alternatively, methods need to be found to alter the product slate of the native coal derived syngas-to-ammonia plants to produce a new mix of products that will restore the economic justification for continued operation of coal-based ammonia fertilizer production plants.
There are a number of well known coal gasification processes for the production of nitrogen-containing synthesis gas such as water gas (WG) or semi-water gas (SWG) where the synthesis gas is employed as feedstreams to reactors for the reduction of nitrogen to ammonia. These feedstreams usually have hydrogen gas to carbon monoxide ratios (H2/CO) in the range of 0.6 to 1.5. Typical of these are the Koppers-Totzek, Winkler and Lurgi processes. It is also well known in the art that these syngas processes can be readily modified to adjust the ratios of hydrogen to carbon monoxide in the product stream to provide a ratio that is more suitable for use in the downstream conversion processes of the installation or more compatible with the overall objectives of the product installation. For example, the desired end-product may be a high purity, high BTU SNG gas, a low BTU clean refinery gas, a syngas for methanol production, or, as in the present case, syngas for ammonia production. It is left to the artisan to determine the more preferred syngas process and/or modification of an installed process to meet the overall product goals of the installation and maximize coal utilization.
The problem addressed herein is the discovery of a process modification of small, coal-based syngas-to-ammonia plant installations that would restore the profitability of those plants in competition with imported ammonia costs and make the continued operation of those native plants both feasible and attractive. The specific objective of the instant invention is the realization of a process integration into a syngas-ammonia facility that can utilize a syngas with variable H2/CO ratios first as a feedstock to produce high value liquid hydrocarbons before ammonia production.
For ammonia plants with natural gas-derived syngas as feedstock, the invention as well provides an option of modifying the product slate from only ammonia to integrated fuels/ammonia co-production to cope with changing market demands and to improve plant economics
It has been discovered that the integration into a syngas-to-ammonia process installation of a Fischer-Tropsch process for the production of liquid hydrocarbons can be effectively achieved to both produce ammonia and a slate of liquid hydrocarbons at a substantial benefit to the overall economics of the integrated process. In particular, the process of the invention includes an integrated process for the production of ammonia-based fertilizer and liquid hydrocarbon products from fossil fuel derived synthesis gas by contacting the fossil fuel with steam and oxygen-containing gas under gasification conditions sufficient to produce a synthesis gas after desulfurization. The low sulfur content synthesis gas comprising nitrogen gas, hydrogen gas and carbon monoxide in a mole ratio of hydrogen to carbon monoxide of between 0.6 to 1 and 5 to 1 is directly passed to a catalytic synthesis process under conditions for the conversion of the carbon monoxide gas and a portion of the hydrogen gas to fuel products comprising substantially liquid hydrocarbon products and light hydrocarbon gaseous products.
The synthesis gas does not have to be processed in a water gas shift unit to convert CO into hydrogen as is normally practiced in ammonia plants since CO is not desirable in ammonia production. While CO is a major reactant for Fischer-Tropsch synthesis which essentially uses up the CO and leaves N2 and part of H2 for ammonia production downstream.
The liquid and gaseous hydrocarbon products and by-product CO2, are separated from both the nitrogen gas and the unreacted portion of the hydrogen gas while the valuable hydrocarbon products are recovered. The nitrogen gas and the unreacted hydrogen gas are recovered as a feedstream to a process for the hydrogenation of the nitrogen whereby ammonia-based fertilizer is produced and recovered.
The basic application of the present invention is the integration of a Fischer-Tropsch (F-T) unit into a fossil-fuell derived syngas-to-ammonia plant such that the synthesis gas (water-gas or semi-water gas) is first converted to liquid hydrocarbons before the production of ammonia by reduction of nitrogen (N2) with the unreacted hydrogen gas effluent from the F-T unit. The water gas shift in the syngas reactor originally used to convert CO into hydrogen before ammonia synthesis is no longer necessary and can be eliminated. When the H2/CO molar ratio of the synthesis gas is 0.8-5.2 most of the CO and part of the H2 in the feed gas are converted to hydrocarbon products in the F-T unit,The off-gas from the F-T system contains significant amounts of unconverted hydrogen, N2 and a small amount of CO. These off-gases are converted to ammonia in a second step of the process in the now integrated fertilizer plant with appropriate N2 balancing. The off gas from the F-T system also contains significant amounts of carbon dioxide (CO2). The carbon dioxide can be used to produce ammonium bicarbonate or urea. Accordingly, the hydrogen gas, carbon monoxide and nitrogen in the feed water gas can be fully utilized.
The hydrocarbon products from the F-T synthesis are highly useful, very clean, straight chain liquid hydrocarbons containing no sulfur and nitrogen. They are useful as household fuels such as LPG, liquid transportation fuels such as naphtha comprising an extra clean gasoline component, and diesel fuels having a very high cetane number in the range of 75-80. The liquid hydrocarbons are also valuable chemical feed stocks. For example, naphtha is an excellent feed stock for ethylene production. The liquid hydrocarbons contain about 50 weight percent of alpha-olefins which are high-value chemicals and various grades of solvents can be made from n-alkanes. A small amount of paraffin wax is also generated.
The Fischer-Tropsch process for the conversion of fossil fuel derived syngas to liquid hydrocarbons is well known in the chemical engineering arts and exemplary descriptions of the application of the process are described in xe2x80x9cCoal Conversion Technologyxe2x80x9d by I. Howard-Smith and G. J. Werner, pp 15-17, 29, 52-5, 58, 77 and 92, which descriptions are incorporated herein by reference. The process is particularly well known for its utilization by Germany during WW II where 10 large plants were constructed to produce hydrocarbon fuels. More recently, coal based F-T plants were constructed in South Africa by the South African Coal, Oil and Gas Corporation Ltd (SASOL) and operated long term for the production of hydrocarbon fuels and chemicals. The F-T process is carried out using iron or cobalt as a catalyst at temperatures between 425 and 700xc2x0 F. and medium pressure between 300 and 800 psi.
In a preferred process of the invention, syngas is generated from coal, preferably employing one or the other of the Lurgi gasification process or the Koppers-Totzek (K-T) or similar gasification process. In the Lurgi process, a fixed bed reactor is employed using oxygen or air and steam at about 20-30 atm and temperatures ranging from 560-620xc2x0 C. In the K-T process, gasification takes place at low pressures and much higher temperatures of about 1480xc2x0 C. Optionally, partial oxidation processes known in the art may be employed to produce the required syngas. The product syngas is desulfurized by methods known in the art such as adsorption on activated carbon or reaction with zinc oxide. Water gas shift is no longer required. H2 to N2 ratio in the off-gas can be adjusted by the balancing (addition) of nitrogen.
The unconverted hydrogen and nitrogen are employed to manufacture ammonia by processes well known in the art. Commercial ammonia production is carried out using a catalytic surface based on metallic iron, typically promoted with other oxides. Operating pressures are high, generally in the range of 2,000 to 5,000 psi.